Heralds of the Sanctuary Doctrine [1517-1843]

We see the the 5th and 6th phases of Church History of the 7 Churches of Revelation (Sardis and Philadelphia) that they were the Age of the Reformation followed by the Age of Revivalism.
That correlates with the white horses of the 4 chariots of Zechariah.

According to Zechariah the red, white and bay horses were sent into

the world. And the man that stood among the myrtle trees answered and said, These are they whom the LORD hath sent to walk to and fro through the earth. Zechariah 1:10.
These white horses were to purify the Church (hence white).

At the end of the 2300 days (years) it (Laodicia) would restore the Sanctuary Truth that had been cast down by the 3rd and 4th phase of Church History (Pergamum and Thyratira) from between the Candlestick in Revelation, represented by the black horses of the chariots in Zechariah.
And out of one of them came forth a little horn, which waxed exceeding great.....and by him the daily sacrifice was taken away, and the place of his sanctuary was cast down....and it cast down the truth to the ground; and it practiced, and prospered.
Then I heard one saint speaking, and another saint said unto that certain saint which spake, How long shall be the vision concerning the daily sacrifice, and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot?
And he said unto me, Unto two thousand and three hundred days; then shall the sanctuary be cleansed. (The Truth of the Light of the Plan of Redemption would be restored at the end of the 2300 days [years] laid out in the Sanctuary).
Daniel 8:9,11-14

The Heralds of the Sanctuary Doctrine Arrive

These white horses, Sardis and Philadelphia, prepared the way for the Sanctuary Doctrine Restored.

For the Church of Sardis, 

AGE OF THE REFORMATION 

Heralded the way of the Outer Court, as the Truth of JUSTIFICATION by Faith was discovered and the fires of it lit, led by Martin Luther.
"The first and chief article is this: Jesus Christ, our God and Lord,

died for our sins and was raised again for our justification. He
alone is the Lamb of God who takes away the sins of the world, and God has laid on Him the iniquity of us all. All have sinned and are justified freely, without their own works and merits, by His grace, through the redemption that is in Christ Jesus, in His blood . This is necessary to believe. This cannot be otherwise acquired or grasped by any work, law or merit. Therefore, it is clear and certain that this faith alone justifies us."
 Martin Luther, "The Smalcald Articles" in Concordia

For the Church of Philadelphia, 

AGE OF REVIVALISM

Prepared the way for the Inner Court, as the Truth of SANCTIFICATION that leads to Obedience was discovered and the fires of it lit, led by John Wesley and his Methodist movement.
“[Full sanctification] is the grand depositum which God has lodged with the people called Methodists; and for the sake of propagating this chiefly God appeared to have raised us up.”
Letter to Robert Carr Brackenbury, September 15th, 1790

For the Church of Laodicea, 

AGE OF THE JUDGMENT

They laid the groundwork for the Restoration of the Sanctuary truth when Laodicia would arrive on scene as the 7th Church and understand the Judgment in the Most Holy Place.

SUMMARY:

SARDIS: 

Restored understanding of JUSTIFICATION in the Outer Court

PHILADELPHIA: 

Restored understanding of SANCTIFICATION in the Inner Court

LAODICEA: 

Restored understanding of the JUDGMENT in the Most Holy Place